Add missing documentation comments

[iosonopersia]

When I wrote documentation comments for this projects, I had not enough time to do it for every module. The following files need to be enriched with comments over classes and functions:

• graph_entity.py
• graph_set.py
• metadata_entity.py
• metadata_set.py
• prov_entity.py
• prov_set.py
• reader.py
• storer.py

Additionally, comments are missing from the following files, which do not contain functions that are publicly accessible:

• query_utils.py
• reporter.py
• support.py